Generally, a forged or cast blank or a round bar material is cut to form a main body, and necessary internal parts are incorporated, screwed or bolted in the main body, thereby assembling ball valves of this class.
These ball valves, however, are expensive because they entail high material costs on account of large numbers of man-hours, high prices of blanks, and large numbers of component parts, and require high process costs for the cutting and the assemblage. They have comparatively great weights as well. Further, since the blanks for processed parts necessitate margins for machining in advance, the costs of materials inevitably increase proportionately.
To cope with these problems, the concept of forming a ball valve by resorting to the technique of press working utilizing a pipe has been proposed. (Refer to JP-A SHO 62-165081, JP-A SHO 63-266276 and JP-A HEI 10-122390.)
However, since the ball valve produced by the press working technique using a pipe has a construction such that it acquires a sealing property by having a ball tightened in a valve seat part, such as a ball seat, which is an elastic member during the assembly of the ball valve, the incorporation of internal components into the pipe necessitates a step of tentative assemblage using the internal components as an integral unit.
The ball valve, therefore, encounters the problem of adding a step of tentative assemblage and a step of dissolving the tentative assemblage, increasing the total number of steps of operation and boosting the cost of production.
Even if the tentative assemblage of internal components should be avoided, manufacturing ball valves uniformly with high accuracy as finished products by the technique under discussion has proved extremely difficult.
The screw connection is prevalently adopted generally for connecting a valve with a small diameter to a pipe. When the ball valve utilizing a pipe is joined to a pipe by the screw connection, since the construction of this ball valve requires a screw part different from the valve seat member to be welded to the terminal part of the body made of the pipe or fastened to the terminal part (expanded diametrically in advance) of the body made of the pipe, this screw connection incurs the problem of complicating the process of production of the ball valve and unduly elongating the interface dimensions of the valve.
This invention which has been developed in view of the problems mentioned above is aimed at providing a ball valve excelling in economy due to the efforts devoted to the reduction of costs, enjoying light weight and high reliability, manifesting high and uniform accuracy, and enabling manufacture of compact finished products having a small interface dimension, and is aimed at providing a method for the production of the ball valve.